Add ambiguous ref modal entrypoints to haml

What does this MR do and why?

see: #408388 (comment 1565303537)

The end goal is to open a modal when a given ref that needs to be displayed is ambiguous (branch and tag with the same name).

This is just the backend part of the work with some front-end elements to facilitate @jerasmus frontend take over.

The implementation with html data attribute was agreed upon in the links here #408388 (comment 1578533983)

Related to #408388 (closed)